This invention relates in general to an improved DME and more particularly to a microprocessor implemented DME that employes a unique method for accumulating and analyzing range data.
A DME calculates slant range distance by measuring the time interval between transmission of a radio signal to a preselected VOR/DME station (hereinafter referred to as a ground station transponder) and reception of a reply signal. Distance may then be indicated in nautical miles on a range/speed/time to station indicator.
In DME systems, the transmitter mounted on the aircraft sends out very short interrogation pulse pairs at irregular intervals. These interrogation pulses are in turn picked up by the receiver of the ground station transponder. Upon receipt of such an interrogation pulse pair, the transmitter of the ground station transponder sends out a pair of reply pulses on a different channel. These reply pulses are received by the airborne receiver which measures the round-trip travel time (time interval between transmission of an interrogation pulse pair and receipt of a reply pulse pair) and converts this measurement into range information.
In system operation, the given ground station transponder will be constantly interrogated by a number of aircraft which are within range and tuned to its channel. The ground station transponder replies to all these interrogations and each aircraft receives the sum total of replies to all of the aircraft. In order to prevent false lock-on under normal operating conditions, it is arranged that each aircraft transmit an interrogation pulse pair that occurs at a rate that is intentionally permitted to "jitter" or vary (within certain limits) in an irregular manner.
The typical DME has two modes of operation, the search mode and the track mode. In the search mode of operation, the DME progressively scans each of the reply pulses to determine which of the received reply pulses are in fact responses to the aircraft's own interrogation pulses. The search operating principal is to locate valid reply pulses by finding the one fixed time interval delay (measured from the DME's own previous interrogation pulse pair) during which a pair of reply pulses is repeatedly received. Since the interrogation pulses from other aircraft are nonsynchronous and random with respect to a given aircraft's interrogation pulses, reply pulses corresponding to such foreign interrogations will not be received by a given aircraft at a regular or slowly changing time delay. Once the DME is locked onto the valid reply pulses, the DME enters the track mode of operation wherein the time delay is converted into range information which is thereafter used to track the progress of the aircraft and keep the DME locked onto the ground station transponder.
Since each DME receives all of the replies produced by a given ground station transponder, the DME must be able to sort through all of the replies and determine which of them are valid, i.e., responses to its interrogation pulse pairs. Conventional DME systems typically employ a second order range data tracking method to detect and filter valid replies. This technique entails the starting of a range counter upon transmission of an interrogation pulse pair and a stopping of the range counter if a reply pulse is received within a set time period or range gate. If a reply pulse is received within the range gate, it is considered to be a valid reply and range data corresponding to this reply is provided to a tracking filter or is used to generate an output which is representative of the distance between the aircraft and the ground station transponder.
A range gate is a time period which is centered on the previous range output of the tracking filter if the DME unit is presently locked onto a ground station transponder. If the DME is not locked onto a ground station transponder, the DME searches for valid replies by initiating a first interrogation and allowing a first reply received following transmission of this interrogation to stop the range counter to provide a range figure which is retained for future reference. This range figure is typically represented by the count state of the range counter upon receipt of the reply pulse. During the next several interrogations, the DME constructs a range gate about this range figure and searches for the receipt of a reply within the range gate. The range gate remains at this position unitl there are a preselected number of excessive interrogations without the receipt of a reply within the range gate. Once this condition occurs, another interrogation is initiated and the range gate is moved to the range figure represented by the first reply following the present range figure. This process continues until the range gate reaches the maximum range figure causing the unit to restart the search process or until a preselected number of valid replies are received before the preselected number of successive interrogations without the receipt of a reply within the range gate. The receipt of a preselected number of replies before the occurrence of a preselected number of successive interrogations without the receipt of reply within the corresponding range gate causes the unit to enter the locked or track mode of operation. In this way, these prior art DME systems search for replies that are not just noise or squitter pulses by searching through and validating each range figure individually.
The above described searching process has a number of inherent disadvantages. One of the most significant drawbacks of this process is that there is no way to determine if the bit is locked onto an echo of the actual reply rather than the actual reply without the use of additional circuitry to monitor the presence of such echos. Another disadvantage of these prior art systems is that only one reply pulse can be evaluated following each interrogation thereby requiring a large number of interrogations to properly lock onto the ground station transponder. Accordingly, these prior art DMEs tend to be rather slow in operation requiring from one to two seconds to lock onto a designated ground station transponder with a high degree of probability. Another disadvantage associated with convention DME units is that the width of the range gate remains constant at all times. In these units, the width of the range gate is determined by a large number of factors including: (1) the interrogation rates; (2) the maximum velocity the unit is designed to track; and (3) the gain constants of the second order filter used in the tracking filter to smooth the range data. To accomodate these factors, the range gate in these prior art DMEs is usually made fairly large. As a result, the probability of an invalid pulse occurring within a range gate is greatly increased, thereby reducing the accuracy of these prior art DME units.